1. Field of the Invention
The present invention relates to the arrangement of a pinion stopper in an engine starter motor.
2. Discussion of the Prior Art
Referring to FIG. 4, there is shown a cross sectional view of a conventional starter, as disclosed in e.g. Japanese Unexamined Patent Publication No. 190967/1989. Referring to FIG. 5, there is shown in an enlarged cross sectional view of a stopper and its surroundings in the starter.
In these Figures, reference numeral 1 designates a the starter as a whole, reference numeral 2 designates an armature, reference numeral 3 designates an armature rotary shaft, reference numeral 3a designates an output shaft, reference numeral 4 designates an overrunning clutch, reference numeral 5 designates a front bracket, and reference numeral 6 designates a switch coil. Reference numeral 7 designates a movable core, reference numeral 8 designates a movable switch contact, reference numeral 9 designates a fixed contact, reference numeral 10 designates a return spring, reference numeral 11 designates a lever, and reference numeral 12 designates a pinion. Reference numeral 13 designates a ring gear, reference numeral 14 designates the stopper, reference numeral 15 designates a stopper fixing ring which is fitted into a groove 3b formed in the output shaft 3a at a leading end of the rotary shaft 3, reference numeral 16 designates a front bearing, reference numeral 17 designates a cap, and reference numeral 18 designates a washer.
Now, the operation of the starter will be explained. When the switch coil 6 is energized, the movable core 7 is attracted to the left to extend the overrunning clutch 4 outwardly or forwardly via the lever 11, causing the pinion 12 to engage the ring gear 13. At the same time, the movable contact 8 engages the fixed contact 9 to enable a large current flow in the electric motor, producing torque to start an engine. At that time, the displacement of the overrunning clutch 4 is restricted because the pinion 11 engages the stopper 14.
On the other hand, when the current is cut off in the switch, the return spring 10 acts on the movable core 7, returning the overrunning clutch to its original position via the lever 11. In that manner, the operation is completed.
In the conventional device, the stopper 14 for restricting the displacement of the overrunning clutch 4, the ring 15 for fixing the stopper 14, and the ring groove 3b with the ring fixed therein are arranged behind the front bearing 16. In such an arrangement, the impact force generated when the pinion 12 engages the ring gear 13 concentrates on the ring groove 3b. As a result, the diameter of the output shaft is subject to a dimensional limitation in terms of strength. The minimum level has been that the number of pinion teeth is 8 in the case of a module of 2.54 (Pd10) and 9 in the case of module of 2.117 (Pd127).
In addition, in order to receive a thrust load in such an arrangement, the output shaft has a stepped portion 3c in abutment with the washer 18 which is press fitted behind the front bearing 16. The front bracket 5 has the cap 17 caulked at a front end thereof for dust sealing.
In general, the size of the electric motor unit is inversely proportional to the gear ratio of a ring gear to a pinion. If the number of pinion teeth is decreased to, e.g. 7 in the case of a module of 2.54 or 8 in the case of a module of 2.117 in order to make the electric motor unit compact, the thickness of the root of the pinion teeth becomes thinner in the conventional structure because the diameter of the output shaft cannot be made smaller due to its strength at the ring groove. Such an arrangement can not ensure the necessary strength required for the pinion, creating a problem in that a decrease in the number of the pinion teeth can not be attained in the conventional structure.